text: Several international standards classify coals by their rank, where increasing rank corresponds to coal with a higher carbon content. The rank of coal is correlated with its geologic history, as described in Hilt's law. 
  In the ASTM system, any coal with more than 69% fixed carbon is classified by its content of carbon and volatiles.  Coal with less than 69% fixed carbon is classified by its heating value. Volatiles and carbon are on a dry mineral free base; heating value is based on the moisture content as mined, but without any free water. 
  The ISO also has a coal ranking system, though its subdivisions do not align with the ASTM standard.